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Title: Heat and Thermodynamics


1
Heat and Thermodynamics
2
  • All matter is in constant motion, so the atoms
    and molecules of all matter have KE.
  • Gases vibrate the most b/c there are no bonds b/w
    the molecules
  • Solids vibrate the least b/c there are the
    strongest bonds
  • This molecular KE gives an object internal energy
    which affects its temperature. Whenever something
    becomes warmer, its because the KE of its atoms
    or molecules has increased.

3
Check Your Understanding
  • Does a solid desk sitting in a classroom have
    kinetic energy?
  • No. The entire desk is not moving so the
    entirety of the desk does not have KE.

4
Check Your Understanding
  • Do the molecules in a solid desk sitting in a
    classroom have kinetic energy?
  • Yes! Even though the desk itself is not moving,
    the individual molecules are constantly moving.
    We do not see the desk moving because they are
    all vibrating back and forth over VERY small
    distances.

5
Temperature
  • Temperature the measurement of the average
    molecular KE of an object.
  • As molecules gain KE, the temperature increases
  • As molecules lose KE, the temperature decreases
  • SI Unit Kelvin (K)
  • Ex 0C 273 K 32F

6
  • Temperature is the measure of the AVERAGE KE of
    the substance, not the total KE.
  • Ex there is more KE in a bucketful of warm
    water than in a cupful of water, but if the
    temperature of the two samples is the same, the
    average KE of the two is the same.
  • There are 3 commonly used temperature scales
  • Fahrenheit
  • Celsius
  • Kelvin

7
Temperature Scale Freezing Point of Water Boiling Point of Water Where it is Used
Fahrenheit 32F 212F In the US
Celsius 0C 100C Most of the world
Kelvin 273 K 373 K In science (SI unit)
8
  • The Kelvin scale and the Celsius scale are the
    same thing
  • A Kelvin is the same size as a Celsius degree
  • If you increase the temperature by 1C, you
    increase it by 1 K
  • If you increase the temperature by 10C, you
    increase it by 10 K
  • Absolute zero the lowest possible temperature.
  • At this temperature all motion ceases, so the
    substance has no kinetic energy.
  • SI Unit Kelvin (k)
  • Ex 0 K -273C

9
Check Your Understanding
  • Which has a greater average KE, a hot cup of
    coffee or swimming pool full of ice cold water?
  • The cup of coffee. As temperature is related to
    the average KE, the one with the higher
    temperature has the greater average KE.

10
Check Your Understanding
  • Which has a greater total KE?
  • The swimming pool of ice cold water. Even though
    the cup of coffee has a higher average KE, the
    swimming pool has more mass and therefore a
    greater overall KE.

11
Heat
  • Heat The energy transfer from one object to
    another because of a temperature difference
    between them.
  • Heat flows from the higher-temperature substance
    into the lower-temperature substance.
  • Heat never flows on its own from a cold substance
    to a hot substance, just like water will never
    flow uphill by itself.
  • Ex You feel cold holding a piece of ice because
    heat flows from your hand to the ice cube

12
  • Thermal Energy the energy resulting from heat
    flow.
  • When heat flows from one object or substance to
    another it is in contact with, the objects are
    said to be in thermal contact.
  • Ex When you hold a piece of ice, you lose
    thermal energy while the ice gains thermal energy

13
Thermal Equilibrium
  • Thermal Equilibrium the state of two or more
    objects or substances in thermal contact when
    they have reached a common temperature.
  • Objects have reached the same temperature
  • Ex when a piece of hot metal is added to a cup
    of cool water, thermal energy flows between them
    until they reach the same temperature of 10
    degrees Celsius

14
Internal Energy
  • Internal Energy the grand total of all energies
    inside a substance.
  • A substance contains internal energy, not heat
  • This energy is due to KE of the moving molecules
    as well as the PE in the bonds holding the
    molecules together.
  • SI Unit Joule (J)
  • Ex An iceberg has a larger internal energy than
    a small cup of coffee

15
Thermal Expansion
  • When the temperature of a substance increases
    this means the average kinetic energy of the
    substance increases, so the molecules move faster
    and further apart.
  • This results in an expansion of the substance.
  • Almost all forms of matter expand when they are
    heated and contract when they are cooled

16
  • Thermometers work off of this concept
  • As the mercury or alcohol heats up, it expands up
    the thermometer
  • When the temperature decreases, the liquid
    contracts down the tube
  • Not all objects have the same rate of thermal
    expansion.
  • Some objects expand more than others
  • Liquids tend to expand more than solids

17
Bimetallic strip
  • A bimetallic strip is two metals welded together,
    steel and copper
  • When the strip is heated, it curves
  • That is b/c the steel side expands at a different
    rate compared with the copper side

18
Check Your Understanding
  • Why is it advisable to allow telephone wires to
    sag when stringing between poles in summer?
  • Because if the lines were pulled tight in the
    summer, when they contract in the winter, they
    would snap.

19
Expansion of Water
  • Water is the exception to thermal expansion
  • Water at the temperature of melting ice contracts
    when the temperature is increased, and will
    continue to contract until it reaches 4C.
  • Water is most dense at 4C
  • This is why ice floats in water
  • The ice is less dense than the water

20
Check Your Understanding
  • What would happen if you put a can of soda in the
    freezer and leave it there overnight?
  • It will explode. As the water goes away from
    4C, the volume increases. As the pressure
    builds from the excess volume, the can will
    eventually explode.

21
Heat Transfer
  • Remember that heat is the transfer of energy
  • There are 3 types of heat transfer
  • Conduction
  • Convection
  • Radiation
  • More than one type of heat transfer can occur at
    the same time, but usually one is more dominate
    than the rest

22
Conduction
  • Conduction energy transfer from one object to
    another when the two are in direct contact.
  • The materials MUST be touching.
  • Heat moves from the warmer object to the cooler
    object.
  • Ex a hot piece of metal burning your hand your
    hand melting a piece of ice

23
  • Materials that conduct (move) heat well are
    called heat conductors.
  • Objects will change their temperature easily
  • Ex metals, glass
  • Wood would be considered a good insulator. An
    insulator delays the transfer of heat.
  • Objects do NOT change their temperature easily
  • Ex Styrofoam, wood, air
  • A poor conductor is a good insulator.
  • Liquids and gases in general are good insulators.
    Porous materials having many small air pockets
    are good insulators as well (the secret behind
    feathers, furs, and wools perceived warmth).

24
Ice HotelIce Hotels and Igloos keep people warm
because ice and snow are very good insulators or
heat.
25
Check Your Understanding
  • Touch a piece of metal and a piece of wood in you
    immediate vicinity. Which one feels colder?
  • Since wood is a poor conductor, the metal feels
    colder b/c it is a better conductor heat easily
    moves out of your warmer hand into the cooler
    metal.

26
Check Your Understanding
  • Which is really colder?
  • If the wood and metal are in the same general
    area, they should have the same temperature (room
    temperature) thus neither is colder.

27
Check Your Understanding
  • Why can fire walkers walk on red-hot coals
    without getting injured?
  • The coals are good insulators, and therefore poor
    conductors, of heat. Even though the coals are
    red hot, they give up very little heat in their
    brief contact with a cooler surface (your foot).
    Things would be different if they were walking
    over red-hot metal shards.

28
Convection
  • Convection heat transfer by movement of the
    atoms themselves from place to place
  • In convection, heating occurs by currents in a
    fluid.
  • Ex heat rises from downstairs to upstairs
    bubbles in boiling water rises to the top and
    rolls back to the bottom of the pot

29
  • Convection occurs in all fluids, liquids and
    gases alike.
  • Convection will NEVER occur in a solid!
  • When the fluid is heated, it expands, becomes
    less dense, and rises. Cooler fluid then moves to
    the bottom and the process continues.

30
Check Your Understanding
  • You can hold your fingers beside a flame without
    harm, but not above the flame. Why?
  • Heat travels upward by air convection. Since air
    is a poor conductor, very little heat travels
    sideways.

31
Radiation
  • Heat from the sun warms the Earths surface after
    passing through the atmosphere.
  • As air is a poor conductor, conduction cannot be
    responsible for this warming. Neither does it
    pass via convection, as convection begins only
    after Earth is warmed.
  • Both forms need molecules to transfer heat.

32
  • Radiation heat transfer by electromagnetic
    waves.
  • Radiation does NOT require a medium to transfer
    heat
  • Ex The sun warms the earth
  • Types of EM waves
  • Radio waves
  • Microwaves
  • Infrared waves
  • Visible light
  • Ultraviolet waves
  • X-rays
  • Gamma rays

33
Absorption of Radiant Energy
  • Absorption and reflection are opposite processes.
  • A good absorber of radiant energy reflects very
    little radiant energy, including light.
  • Because of this, a good absorber appears dark.
  • A perfect absorber reflects no radiant energy and
    appears perfectly black. No visible light is
    reflected.
  • Ex Your eyes pupil

34
Check Your Understanding
  • If you paint the inside of a box white and cut a
    hole on one side to see inside, the hole appears
    black. Why?
  • Radiant energy that enters an opening has little
    chance of leaving before it is completely
    absorbed. Thats why holes appear black.

35
Check Your Understanding
  • Why are most coffee mugs painted white on the
    inside?
  • B/c white is good reflector of radiant energy
    (light). You want the radiant energy to be
    reflected back into your coffee, keeping it
    hotter for a longer time.

36
Emission of Radiant Energy
  • Good absorbers are also good emitters
  • poor absorbers are poor emitters
  • poor absorbers are good reflectors
  • Light colored objects in contact with dark
    colored objects eventually reach thermal
    equilibrium. The warmer, darker object must give
    up (emit) radiant energy to the cooler, lighter
    object.

37
Check Your Understanding
  • Is it more efficient to paint a heating radiator
    black or silver?
  • Black. The radiator painted silver would be a
    poor emitter and poor absorber or energy. Black
    would increase the contribution of heat from the
    radiator.

38
Thermodynamics
  • Thermodynamics the study of heat and its
    transformation into mechanical energy.
  • The foundation of thermodynamics is the law of
    conservation of energy and the fact that heat
    flows from a warmer object to a cooler object

39
1st Law of Thermodynamics
  • The First Law of Thermodynamics whenever heat
    is added to a system, it transforms to an equal
    amount of some other form of energy.
  • Conservation of energy energy cannot be created
    nor destroyed, just change from one form to
    another
  • Ex Food changes from chemical energy to body
    heat and/or kinetic energy

40
Object/ Process Energy Conversions
Automobile Engine Chemical Kinetic
Heater/Furnace Chemical Heat
Hydroelectric Gravitational Potential Electrical
Solar Optical Electrical
Nuclear Nuclear Heat, Kinetic, Optical
Photosynthesis Optical Chemical
Food Chemical Heat, Kinetic
Battery Chemical Electrical
41
  • When energy is added to a system (any group of
    atoms, molecules, particles, or objects), this
    energy does one or both of two things
  • increase the internal energy of the system if it
    remains in the system
  • does external work if it leaves the system
  • Equation
  • heat increase in external work done
  • added internal by the system
  • energy

42
Check Your Understanding
  • If 10 J of energy is added to a system that does
    no external work, by how much will the internal
    energy of that system be raised?
  • 10J
  • If 10J of energy is added to a system that does
    4J of external work, by how much will the
    internal energy of that system be raised?
  • 10J 4J 6J

43
Second Law of Thermodynamics
  • The second law of thermodynamics heat will
    never flow by itself from a cold object to a hot
    object.
  • In order for heat to flow from a cold object to a
    hot object, work must be put into it
    (refrigerators work off of this idea)
  • Ex heat flows from your hand to a piece of ice
    (from hot to cold)

44
Entropy
  • Natural systems tend to proceed toward a state of
    greater disorder.
  • Entropy the measure of the amount of disorder.
  • As disorder increases, entropy increases.
  • The second law states that for natural processes,
    in the long run, entropy always increases.
  • Ex liquids evaporating, ice melting, a messy
    room
  • You cant get more energy out of a system then
    what you put in it.
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